This application claims the benefit of priority to Japanese Patent Application 2000-181427, filed on Jun. 16, 2000.
1. Field of the Invention
The present invention relates to a method for driving a paper-feeding stepping motor in a thermal printer, and more particularly, to a method for driving a paper-feeding stepping motor in a thermal printer which performs so-called dynamic division printing in which a desired image is recorded line by line by divisionally energizing heating elements so that the number of the heating elements to be simultaneously energized for each line is less than or equal to a predetermined number.
2. Description of the Related Art
Conventionally, stepping motors are used as carriage motors for driving carriages or paper-feeding motors for feeding recording paper in printers because the rotation angle and speed of the motors are determined in proportion to the input pulse number and the input pulse speed, the starting and stopping characteristics are far superior, and a high responsiveness and a high power can be obtained.
A line-type thermal printer will be described as an example in which such a stepping motor is used as a stepping motor for feeding recording paper. In a line-type thermal printer, a long platen roller is rotatably supported between a pair of side frames, and a line thermal head is supported in a printer body by a support lever so as to be moved closer to and further apart from the platen roller. The line thermal head has, in its rear side, head-pressing springs. The line thermal head also has an array of heating elements arranged in a longitudinal direction. An energization control section is electrically connected to the heating elements so as to selectively control the energization of the heating elements based on the recording data.
On the other hand, a paper-feeding stepping motor is mounted in the printer body. A delivery roller is linked with a driving shaft of the paper-feeding stepping motor via a transmission gear train so as to take thermosensitive sheets (thermal recording sheets) out of a paper tray and to supply the sheets between the platen roller and the line thermal head. A feeding control section is connected to the paper-feeding stepping motor via a motor driver.
In order to perform recording by the line-type thermal printer having such a configuration, first, the paper-feeding stepping motor is driven and the delivery roller is rotated, thereby feeding thermal recording sheets one by one from the paper tray, and supplying the thermal recording sheets between the platen roller and the line thermal head. When a thermal recording sheet is conveyed to a recording start position, the line thermal head is pressed against the platen roller with the thermal recording sheet and the ink ribbon therebetween, the energization control section selectively energizes the heating elements based on the recording data, and the thermal recording thereby develops color. When the recording of the first line is completed, the paper-feeding stepping motor is driven, and the recording of the second line is started based on the recording data. In this way, the recording is continued to the final line.
A description will now be given of the energization control of the heating elements of the line thermal head by the energization control section. Hitherto, when energizing the heating elements, a so-called dynamic division printing is performed in order to reduce the power consumption, in which the heating elements are energized in arbitrary divisions so that the number of heating elements to be simultaneously energized for one line is less than or equal to a predetermined number, as shown in FIG. 7.
According to such dynamic divisional printing, the power to be supplied at one time can be reduced. Moreover, since the number of divisions is not fixed, but is set to be best-suited to the required number of heating elements to be energized for printing each line, the printing speed does not become excessively slow.
In the conventional method for driving the paper-feeding stepping motor of the thermal printer, however, since the number of divisions for energization is set for each line, it often varies from line to line. The paper-feeding stepping motor is driven by applying active pulses of a fixed voltage regardless of the number of divisions. For this reason, in the case of a line which is recorded in a large number of divisions, the torque excessively increases and this produces noise.
That is, as the number of divisions increases, the energizing time necessary for the recording of one line increases, and active low and high outputs corresponding to the phases are applied to a driving line for controlling the paper-feeding stepping motor for a long period. For this reason, force for reducing the rotational inertia force is applied, and noise is thereby produced.